Increasingly so, today's communications uses optical data transmitted through, for instance, a fiber optic cable. At the receiving end of a fiber optic link, a photodetector receives the light which may be short wavelength and/or long wavelength light and generates an electrical current proportional to the intensity or power of the light. This photocurrent is then conditioned and coupled to a transimpedance amplifier which converts the photocurrent into a proportionally scaled output voltage signal. The photoreceiver, may be comprised of the photodetector and the transimpedance amplifier, can be packaged into a TO can. On the other end of the TO can are electrical contact pins to transmit electrical data output derived from the optical input and power and ground pins. The output of the photoreceiver can be input into a host data processing system, such as a computer via the electrical contact pins.
Just as electrical data can be transmitted at different speeds, so is optical data transmitted at different speeds. Autonegotiation is the process by which two transceivers capable of multiple bit-rate operation communicate according to a defined protocol to establish the highest data transfer rate possible between the two transceivers. A common example of an autonegotiation occurs between computer modems, where the devices “negotiate” until the highest possible baud rate is established.
Autonegotiation in optical networking standards has become a very hot topic because new multi-gigabit per second (Gb/s) standards are being developed. There is, however, a strong desire from end users for the newer high speed transceiver modules to interoperate with the large installed base of devices capable of only lower-speed operation. One excellent example can be found in the Fibre Channel market where there is a large effort to implement an autonegotiation protocol to ensure that the faster 2.125 Gb/s modules can successfully communicate with the older, widely deployed 1.0625 Gb/s modules.
One of the main transceiver module parameters that is an obstacle to multiple bit rate operation is the bandwidth of the receiver path in an optical transceiver. Transmission errors arise when the optical signal from a lower speed module is presented to a high-bandwidth receiver in a high-speed module. As mentioned, there are many older Fibre Channel optical transceivers that operate at 1.0625 Gb/s and utilize lasers with low relaxation oscillation frequencies (ROF) that introduce substantial overshoot and undershoot in the “1” level of transmitted bits. When one of these low ROF lasers is used as the input to a 2.125 Gb/s module, it will not be sufficiently attenuated and can result in an incorrectly received bit if the bandwidth of the receiver in the higher speed module is too high.
There are two known autonegotiation methods to alleviate the above problems. The first uses clock recovery techniques to detect the data rate wherein the fundamental clock frequency of an incoming data stream is detected and regenerated. Circuits that perform this function are widely available and the technology is mature. Nevertheless, a reference clock signal is required along with a clock recovery circuit in order to determine the actual frequency of the recovered clock and therefore the bit rate of the transmitted data. The other known method to alleviate this problem is to implement a switchable bandwidth receiver wherein the bandwidth of the receiver is lowered for low-speed operation and maintained at it's highest value for high-speed operation, such as disclosed in U.S. Ser. No. 09/574,239 now U.S. Pat. No. 6,862,322 entitled Switchable-Bandwidth Optical Receiver, filed 19 May 2000, which is commonly owned by the assignee herein and which is incorporated by reference in its entirety. The described method provides a control signal to an optical transceiver that modifies certain aspects of the transceiver's performance to allow operation at multiple bit rates. Even a switchable-bandwidth receiver as described, however, requires a control signal to determine when the receiver's bandwidth needs to be modified.
There is thus a need in the industry for a low cost, easy-to-implement, and fully self-contained data rate detector and a method of data rate autodetection that allows selection of the data rate of an incoming digital signal without an external control signal or complicated clock recovery schemes.
Other objects, features, and characteristics of the invention; methods, operation, and functions of the related elements of the structure; combination of parts; and economies of manufacture will become apparent from the following detailed description of the preferred embodiments and accompanying figures, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures.